This invention relates to a driving circuit of a grating for rotating the grating of a monochromator and, more particularly, to a driving circuit of a grating in which, when wavelength data is input, angle data is calculated from the wavelength data, and the grating is caused to rotate on the basis of this calculated result.
Conventionally, in one type of monochromator optical equipment, in the case of detecting the wavelength distribution or the like of light as shown in FIG. 1, a incident light 3 is split into spectra by a grating (diffraction grating) 1 rotatably provided at a predetermined portion of the main body of the monochromater (not shown). A light component having a desired wavelength is thus taken from the incident light 3 by a slit apparatus (not shown) and focusing it in a light receiver apparatus (not shown).
With such a grating system, however, when the incident light 3 is provided thereto with only an angle .alpha. of inclination from a normal line 2, which is perpendicular to a lattice plane 1.sub.a of the grating 1, a primary light 4 having a wavelength .lambda., represented by the equation EQU .lambda.=d(sin .alpha.+sin .beta.) . . . (1)
is emitted in a direction with an angle .beta. of inclination from the normal line 2, wherein d indicates a lattice constant of the grating 1. On the other hand, in this case, a difference angle .gamma. between the incident light 3 and the primary light 4 shown in FIG. 1 is determined dependent upon the location of a mirror (not shown) to supply the incident light 3 to the grating 1 and the location of a mirror (not shown) which receives the primary light 4. Therefore, assuming that .beta.-.alpha.=.gamma. (constant) and by substituting this for equation (1) and further by modifying the equation thus obtained, ##EQU1## can be derived. In this equation (2), since 2d.times. cos ##EQU2## is a constant, this is substituted by a constant A. Also since the angle ##EQU3## is the angle between a normal line 2 and an axis 5 which divides the angle .gamma. into two, this is substituted by a variable .theta.; thus, equation (2) will be EQU .lambda.=A.multidot.sin .theta. . . . (3).
As will be understood from equation (3), to emit the primary light 4 of the wavelength .lambda. from this grating 1 when the incident light 3 is supplied to the grating 1, it is necessary to rotate the grating 1 by only the angle .theta. ##EQU4## corresponding to the wavelength .lambda..
FIG. 2 is a diagram showing one example of a sine bar as one of the devices to perform such wavelength/angle conversion. As illustrated in this diagram, the sine bar 12 is constituted in a manner such that a rod member 9 which abuts a movable member 8 is rotated around an axis 10 in association with the movement of the movable member 8 in the X direction by means of a motor or the like (not shown), and at the same time, a grating mounting plate 11 fixed to this axis 10 is also rotated. Therefore, when the amount of travel S of the movable member 8 is small, the relation EQU S=R.multidot.sin .theta. . . . (4)
is satisfied between the travel amount S and the rotational angle (.theta..sub.1 -.theta..sub.2) of the grating mounting plate 11. In this case, R indicates a distance from the axis 10 to the contact point of the rod member 9 and movable member 8, and .theta. represents the difference angle between the angle .theta..sub.1 before deformation and the angle .theta..sub.2 after deformation. As can be seen from a comparison of equations (3) and (4), if the movable member 8 can be moved by only the amount S ##EQU5## corresponding to the wavelength .lambda., the primary light 4 of the wavelength .lambda. can be emitted from the grating 1.
However, since the amount of change of the arm length R of the rod member 9 increases with an increase in the travel amount S, such a sine bar 12 cannot cover a wide wavelength range. Also, even when the travel amount S lies within a narrow range, the arm length R slightly changes and the rod member 9 and the movable member 8 are mutually worn out due to contact with each other, so that a high degree of accuracy cannot be obtained. Further, such a sine bar 12 cannot be miniaturized due to the structure thereof, so that there is an inconvenience such as an increase in mounting space for the grating driving apparatus.